1. Field of the Invention
The present invention relates to a de-interleaver used for error correction at a receiver in radio communications, a de-interleaving method, and a mobile communication terminal having the de-interleaver.
2. Description of Related Art
Interleaving is performed in radio communications in order to spread burst errors generated by phasing or the like. When transmitting encoded data, a transmitter performs interleaving that reorders a data stream to be transmitted. Then, a receiver performs de-interleaving that rearranges the received data stream back in the original order and decodes the data stream after that. This process converts burst errors into spread random errors, thereby increasing the error correction capability.
Interleaving process within wideband code division multiple access (W-CDMA) is defined in 3rd Generation Partnership Project (3GPP)-TS25.212. It specifies that a transmitter performs first interleaving on a data stream where one transport channel (TrCH) has been encoded. The first interleaving process changes the order of data bits in a transmission time interval (TTI) frame, which is a unit of transmission over a TrCH. Further, TS25.212 specifies to perform rate matching prior to first interleaving and to segment the TTI frame into radio frames (10 ms), which is a unit of radio transmission, after first interleaving.
Rate matching involves the process to reduce the number of bits contained in a TrCH. The processing mode for reducing the number of bits is called compressed mode. The compressed mode is to turn off transmissions temporarily to create transmission gaps when transmitting data from a transmitter (e.g., a base station) to a receiver (e.g., a mobile communication terminal). The transmission idle time obtained by the transmission gaps is used to measure the frequency of a handover target channel upon handover to a different frequency.
One method for creating transmission gaps in the compressed mode is called compressed mode by puncturing. Puncturing creates transmission gaps by decimating data bits on a TrCH. If rate matching specifies the implementation of puncturing, a process to remove data bits corresponding to a transmission gap from a TrCH is performed.
The first interleaving process that is performed on the TrCH after puncturing is performed as follows. First, the process inserts redundant bits (P bits) of the same number as the bits reduced by puncturing into a TTI frame which is not interleaved. Then, it interleaves the data stream where the P bits are inserted. The P bits are inserted so that they are positioned at the beginning of radio frames when the interleaved TTI frame is segmented into radio frames.
After first interleaving, the process segments the TrCH into radio frames, subsequently performs multiplexing which is specified in TS25.212, and then transmits the radio frames to a receiver. The P bits positioned at the beginning of the radio frames are removed prior to the transmission so that transmission gaps are created between the radio frames to be transmitted.
On the other hand, when performing de-interleaving in a receiver, it is necessary to implement the reverse process to the first interleaving process performed in the transmitter. This process is called first de-interleaving. FIG. 8 shows a configuration example of a conventional first de-interleaver 800 when puncturing is implemented. The first de-interleaving operation is described with reference to FIG. 8. Specifically, a radio frame concatenation and P bit insertion section 82 reads received radio frames from a radio frame buffer 81 and inserts a P bit at the beginning of the radio frame.
When inserting the P bit, the number of P bits to be inserted into the TTI frame is read out from a P bit information table 86. The number of P bits to be inserted is the same as the number of P bits inserted in the first interleaving process at a transmitter. The number is notified by a base station in the transmitter and stored in the P bit information table in advance.
Then, a de-interleaving section 83 performs de-interleaving on the frame data to which the P bit has been inserted according to permutation rules set by a permutation rule table 87. The permutation rule table 87 retains the permutation rules predetermined according to the length of the TTI frame. The de-interleaving section 83 performs de-interleaving according to the permutation rules stored in the permutation rule table 87 and stores the de-interleaved frame to a TTI frame buffer 84.
After that, a P bit removal section 85 removes all the P bits from the data stored in the TTI frame buffer 84 and sends the data to a decoding block in a subsequent stage.
De-interleavers and de-interleaving methods which reduce the memory buffer capacity required for de-interleaving have been proposed. They are described in Japanese Patent No. 3239872 and Japanese Unexamined Patent Publication No. 2004-147240, for example.
However, the present invention has recognized that, in the conventional first deinterleaver 800 described with reference to FIG. 8, the TTI frame buffer 84 that stores de-interleaved TTI frames requires as much capacity as storing all the TTI frames where P bits are added. It is therefore necessary to have a buffer memory of a large capacity to store P bits, which are merely redundant bits.